Abstract
Calculations of equilibrium temperatures under a water vapor canopy which minimizes the greenhouse effect show that if the solar constant was less than 25% of today’s value the surface temperature would be livable. In fact, for a solar constant approaching 1% of today’s value it appears that a dense water vapor canopy would be necessary to avoid the entire atmosphere, including the oxygen and nitrogen, from precipitating to the surface as snow. If appropriate conditions can be demonstrated which justify the assumption of a much lower solar constant than typically studied, these calculations could revive consideration of an early earth covered by a water vapor canopy.